Copper-catalyzed direct amination of the superficial graphenic domains of multi-walled carbon nanotubes

Abstract

This paper reports copper-catalyzed direct functionalization of multi-walled carbon nanotubes (MWCNTs) with polyamines. Notably, and contrary to previous reports on direct amination of carbon nanotubes, the yield of this catalytic procedure is independent of the degree of defects on the MWCNT graphenic surface, which results in the efficient functionalization of both very defective and highly regular MWCNTs. This methodology allows us to obtain very high degrees of surface functionalization (much higher than those previously reported) under mild conditions (refluxing toluene). Hybrid materials containing up to 30.9 wt% hyperbranched polyethyleneimine (HBPEI, M_n = 1800) were prepared following this procedure. The most efficient catalysts are Cu(I) salts, in particular the iodide, while Cu(II) and Ni(II) salts result in poorer functionalization degrees. The method proposed is useful to functionalize MWCNTs with polyamines of diverse molecular weight: from ethylenediamine, diethylenetriamine or tris(2-aminoethyl)amine to HBPEIs of M_n = 600, 1800 and 10 000. The structural analysis of the amine-functionalized products (hybrids) supports the covalent grafting of amine nitrogen to the MWCNT walls. The characteristics of these hybrids open many possibilities for potential uses as supports for a wide range of catalytic metal/metal oxide nanoparticles. Moreover, they show improved dispersibility in water, their electronic behavior exhibits a semiconductor character and they also present a good ability to trap carbon dioxide.